In a liquid chromatograph mass spectrometer (LC-MS) using a mass spectrometer as the detector for a liquid chromatograph (LC), an ion source which employs an atmospheric pressure ionization method, such as an electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI), is used in order to ionize a liquid sample eluted from a column of the LC while gasifying the same sample. For example, in an ESI ion source, a high voltage (e.g. ±several kV) having the same polarity as the ion to be analyzed needs to be applied to the tip of the nozzle from which the liquid sample is sprayed.
In such a mass spectrometer, when the detection of a positive ion and that of a negative ion is alternated with a short period of time, the polarity of the applied high voltage needs to be switched according to the polarity of the ion. For this operation, a high-voltage power supply device capable of switching the polarity of one system of output voltage is used. As a high-voltage power supply device of this type, a device described in Patent Literature 1 has been known. This high-voltage power supply device has a positive voltage generation circuit and negative voltage generation circuit, each of which includes a DC-DC conversion circuit using an isolation transformer, with one resistor parallel-connected to the positive output terminal of the positive voltage generation circuit and another resistor parallel-connected to the negative output terminal of the negative voltage generation circuit, and the two resistors connected to each other in series. Using one end of the series-connected resistors as the reference end, a high positive or negative voltage is extracted from the other end.
In this high-voltage power supply device, the high voltage is generated in each of the positive and negative voltage generation circuits by supplying a predetermined drive signal from a control circuit to a switching element included in each of the positive and negative voltage generation circuits. When the high voltage output is switched between the positive and negative polarities, the positive and negative voltage generation circuits are controlled so that the output voltage of one circuit changes from a positive high voltage +HV to zero while that of the other circuit simultaneously begins to change from zero to eventually converge to a negative high voltage −HV after overshooting this level. This intentional overshooting of the voltage which is being raised from zero is aimed at reducing the amount of time required for the positive/negative switching of the polarity.
As a high-voltage power supply device capable of an even quicker positive/negative switching of the polarity, a device disclosed in Patent Literature 2 has been known. In this high-voltage power supply device, the resistors respectively connected in parallel to the positive output terminal of the positive voltage generation circuit and the negative output terminal of the negative voltage generation circuit in the device described in Patent Literature 1 are replaced by switch circuits consisting of FET or similar devices. A circuit consisting of two resistors connected in series is connected in parallel to each of the switch circuits. A signal extracted from the connection point of these two resistors on the positive side is sent as the on/off drive control signal to the negative-side switch circuit, while a signal extracted from the connection point of the two resistors on the negative side is conversely sent as the on/off drive control signal to the positive-side switch circuit.
In high-voltage power supply device, for example, consider the situation where the positive voltage generation circuit is operated to make the high positive voltage +HV appear at its output terminal, while the negative voltage generation circuit is in effect inactive, with the voltage at its output terminal at approximately zero volts. In this situation, a voltage which equals the high voltage +HV divided by the ratio between the resistance values of the two resistors at the positive output terminal is generated at the connection point of these two resistors and given to the switch circuit on the negative side. Consequently, this switch circuit turns on, and the conduction state is established between the output terminals of the negative voltage generation circuit. From this state, when the polarity of the voltage is switched from positive to negative, the positive voltage generation circuit is deactivated, while the negative voltage generation circuit is activated. When the voltage at the output terminal of the positive voltage generation circuit decreases to a predetermined level, the switch circuit on the negative side turns off. Meanwhile, the voltage at the output terminal of the negative voltage generation circuit increases, and this time, the switch circuit on the positive side turns on. Consequently, the electric charges remaining at the output terminal of the positive voltage generation circuit are rapidly discharged through the switch circuit, so that the output voltage of the positive voltage generation circuit rapidly decreases to zero.
In this manner, when the positive/negative switching of the polarity is performed, the two switch circuits respectively provided at the positive and negative output terminals function so as to induce the forced discharge of the electric charges remaining at the output terminal on the side corresponding to the polarity which is about to change to zero. This leads to a quick decrease in the voltage which is about to change to zero, so that the positive/negative switching of the polarity is more quickly achieved.
Using such a high-voltage power supply device as the voltage supply source for a nozzle of an ESI ion source makes it possible to perform an LC/MS analysis while alternately switching a positive ion measurement mode and negative ion measurement mode with a short period of time. This is extremely useful, in particular, for a simultaneous multi-component analysis or similar analyses since both the compounds which easily turn into positive ions and the compounds which easily turn into negative ions can be exhaustively detected.
In recent years, an even greater improvement in the sensitivity of mass spectrometers has been demanded, in particular, for such purposes as the quantitative determination of trace amounts of components. Conventionally, it has not been a common practice to finely control the value itself of the voltage applied to the nozzle in an ESI ion source. However, it is commonly known that appropriately adjusting the voltage applied to the nozzle according to such factors as the properties of the target compound improves the ionization efficiency and provides a higher level of ion detection sensitivity than applying a fixed level of voltage. The optimum voltage to be applied to the nozzle depends not only on the kind of compound but also the kind of dilution solvent in the sample, the kind of mobile phase, supply conditions of the mobile phase and other factors. Accordingly, in recent years, attempts have been made to improve the detection sensitivity for each compound in a SIM (selected ion monitoring) or MRM (multiple reaction monitoring) measurement by adjusting the voltage applied to the ESI ion source for each target ion in the SIM measurement or each target MRM transition (which is a combination of the precursor ion and the product ion).
In an MRM measurement performed in a simultaneous multi-component analysis, it is normally necessary to change the MRM transition within a short period of time of a few msec to several tens of msec. However, attempting to change the value of the voltage applied to the nozzle for such a change of the MRM transition causes the following problem:
Not only the aforementioned high-voltage power supply device but also any high-voltage power supply device of the same type normally has a capacitor connected to its output terminal to smooth the output voltage. In the phase of increasing the voltage value (absolute value of the voltage), the voltage rises at high rates since the capacitor can be quickly charged by increasing the output current. By comparison, in the phase of decreasing the voltage value (absolute value of the voltage), the electric charges accumulated in the capacitor need to be discharged through a channel including output resistors and other elements. Since the discharge rate depends on the time constant, the rate of decrease in the voltage is considerably lower than that of the increase in the voltage. In the high-voltage power supply device described in Patent Literature 2, when the polarity of the output voltage is changed, the switch circuit provided at the output terminal effectively works to discharge the accumulated electric charges. However, when the output voltage is decreased without changing its polarity, the switch circuit does not work, so that the voltage requires time to change. For example, in one example of the high-voltage power supply device described in Patent Literature 2, a voltage-changing process for increasing the voltage from +2 kV to +5 kV requires 1 to 5 msec, while a voltage-changing process for decreasing the voltage from +5 kV to +2 kV requires 10 to 50 msec, which is approximately ten times slower than in the voltage-increasing process.
For example, in such applications as a simultaneous multi-component analysis of agricultural chemicals residues, the cycle of sequentially performing MRM measurements for the ions originating from a plurality of target compounds is repeated. If a considerable amount of time is required for the changing of the applied voltage as in the previously described case, it will be necessary to shorten the data acquisition time (or so-called “dwell time”) or limit the number of MRM transitions to be concurrently subjected to the measurement. In the former case, the detection sensitivity will be sacrificed. In the latter case, fewer compounds can be simultaneously subjected to the measurement, and in some cases it may be necessary to perform the measurement multiple times for the same sample.